The present invention relates to a substrate material which is an intermediate for a printed circuit board, a process for production of the substrate material, and an intermediate block used in production of the substrate material.
Printed circuit boards have, at one side, slots for integrated circuit and terminals for connecting various electronic parts and, at another side, printed conductive paths for connecting the parts, and have been used in a large amount as a key member of electronic appliances.
FIG. 6 is a perspective view showing a printed circuit board. In FIG. 6, a substrate material 1 comprises a sheet-shaped material composed of an insulating material (e.g. epoxy resin or glass) and a conductive metal 2 formed in the sheet-shaped material by plating or the like so as to electrically connect the two surfaces of the sheet-shaped material; on the two surfaces of the substrate material 1 is laminated a photoprocess layer 3 (a conductive layer) in which a predetermined conductive pattern (circuit) is formed; above and below the photoprocess layer 3 are formed terminals and conductive paths 4 by printing or the like; thereby, a printed circuit board is constituted.
The substrate material 1 used in printed circuit boards has heretofore been produced, for example, by producing a sheet-shaped material composed of an insulating material (e.g. epoxy resin or glass), making through holes (for electrical connection) at the predetermined positions of the sheet-shaped material by drilling, filling the through holes with a conductive metal (e.g. copper) by means of plating or the like, and sealing the through holes with a sealant.
In the above production, however, drilling in the shaped material generates chips and has had a fear of producing defective products, and plating has had a high possibility of generating cracks at the periphery of the substrate material, inviting poor electrical connection. Further, in the drilling, the achievable ratio of through hole length (substrate thickness)/hole diameter is at best about 5 and, in, for example, a substrate material having a thickness of 1 mm, the lower limit of hole diameter is about 0.2 mm. In order to obtain a printed circuit board of high density, a smaller hole diameter is preferred; however, such a hole diameter has been difficult to achieve by drilling.
There was proposed a circuit board obtained by inserting, into a frame, electric wires composed of Ni, Co or the like, pouring thereinto a molten insulating material composed of an epoxy resin or the like, curing the insulating material, and cutting the resulting material at a plane perpendicular to the metal wires to allow the two surfaces of the cut material to have electrical connection to each other (see JP-A-49-8759).
In this circuit board, however, the epoxy resin or the like used causes, in curing, a volume shrinkage of about 2 to 3%, which has impaired the dimensional accuracy of through hole pitches, etc. This has been a big drawback because dimensional accuracy is very important in high-density printed circuit boards.
Further in this circuit board, since no attention is paid to the difference in thermal expansion between the substrate material and the conductive layer [photoprocess layer] laminated on one or both surfaces of the substrate material, peeling may occur between the substrate material and the conductive layer owing to the impact and/or temperature difference during use. There has also been a fear of peeling between the insulating material and the metal wires.
Hence, in view of the above-mentioned conventional problems, the present invention has an object of providing a substrate material for printed circuit, which ensures good electrical connection and possesses controlled thermal expansion so as to eliminate the peeling during use between the substrate material and the conductive layer and between the insulating material and the metal wires; and a process for production of such a substrate material for printed circuit.
Another object of the present invention is to provide a substrate material for a printed circuit, which can give a printed circuit board of high density and has excellent dimensional accuracy; and a process for production of such a substrate material for printed circuit.
Still another object of the present invention is to provide an intermediate block from which the above substrate material for a printed circuit can be produced easily and efficiently.
According to the present invention, there is provided a substrate material for a printed circuit, comprising a sheet-shaped composite material composed of a plastic and a ceramic and conductive metal wires fixed in the composite material at given pitches, wherein the two surfaces of the substrate material have electrical connection to each other via the metal wires.
The substrate material of the present invention preferably has a copper layer on one or both sides because it enables easier pattern formation. The content of the ceramic in the composite material is preferably 40 to 90% by volume because it can make smaller the volume contraction during curing. It is preferred from the standpoint of prevention of peeling of the metal wires from the composite material that in the composite material, the plastic and the ceramic have each been subjected to a coupling treatment and further that the metal wires and the composite material are bonded to each other by a coupling agent.
It is desirable that the composite material is composed of an epoxy resin and a glass fiber cut into a predetermined length or glass beads, because the resulting substrate material has no anisotropy in thermal expansion and can have an intended strength.
Also in the substrate material of the present invention, it is preferred that the metal wires used have an aspect ratio (length/diameter) of 8 or larger, that the metal wires are fixed in the sheet-shaped composite material at pitches of 1.1 mm or smaller, and that the metal wires have a diameter (corresponding to the diameter of through holes or via holes) of 0.2 mm or smaller, because a high-density printed circuit board is obtainable.
The substrate material of the present invention can be allowed to show isotropic thermal expansion and have a thermal expansion coefficient of 5 to 30 ppm/xc2x0 C.; the composite material can be allowed to have a thermal expansion coefficient at least equal to that of the metal wires; and the difference between the two thermal expansion coefficients can be made as small as 1 to 10. Thereby, the reliability in the temperature history of steps can be made very high.
According to the present invention, there is also provided an intermediate block used in production of a substrate material for a printed circuit, comprising a composite material composed of a plastic and a ceramic and conductive wires fixed in the composite material at given pitches, wherein the content of the ceramic in the composite material is 40 to 90% by volume and wherein the metal wires extend lineally from one surface of the intermediate block to other surface of the intermediate block facing the one surface and project from the two surfaces.
In the substrate material for printed circuit and the intermediate block therefor, both of the present invention, the metal wires are composed preferably of any one metal selected from copper, copper alloys, aluminum and aluminum alloys and, in view of the standpoints of conductivity, abrasion resistance, flexibility, oxidation resistance, strength, etc., more preferably of beryllium copper.
According to the present invention, there is also provided a process for producing a substrate material for a printed circuit, which comprises stretching, in a mold, conductive metal wires at given pitches, then pouring, into the mold, a composite material composed of a plastic and a ceramic, curing the composite material, thereafter slicing the resulting material in a direction approximately perpendicular to the direction of the metal wires.
In the above production process, the thermal expansion coefficient of the material constituting the mold is preferably larger than that of the metal wires, and the metal wires have a diameter of preferably 0.2 mm or smaller and are fixed in the mold at pitches of preferably 1.1 mm or smaller.